1. Field of the Invention
The present invention relates to an inchworm type of actuator which moves by small degrees or in little increments by one pitch relative to a guide member. It relates, in particular, to an inchworm type of actuator which is suitable as a driving apparatus for changing the position a workpiece receiver on a jig depending on the kind of workpiece. In this specification, the above-described movement by small degrees or in little increments is called "inching" or "an inching movement."
2. Description of the Related Art
As an inchworm type of actuator, there is conventionally known the following one in Japanese Published Unexamined Patent Application No. 32631/1972. Namely, a pair of casings which are telescopically connected via a hydraulic jack are slidably engaged with a bar-like guide member. Each of the casings has assembled or built therein a hydraulic brake unit which locks and unlocks each of the casings relative to the guide member. While keeping the brake unit in one of the casings in a locked condition and the brake unit in the other of the casings in an unlocked condition, the other of the casings is inched (or moved in an inching manner) by the extension operation of the hydraulic jack. Then, while keeping the brake unit in the other of the casings in the locked condition and the brake unit in said one of the casings in the unlocked condition, said one of the casings is inched towards the other of the casings by the contraction operation of the hydraulic jack. By repeating the above operations the actuator is inched by one pitch along the guide member.
In order to improve the control response of the actuator, it is desired to mount on the casing valves which control the supply and discharge of the hydraulic oil to and from the hydraulic lack and the brake units so as to shorten to the maximum extent possible the length of the fluid passages between the hydraulic jack and a valve for the hydraulic jack between the brake units and valves for the brake units.
In the above-described conventional actuator, the hydraulic jack can be integrated with one of the casings. Therefore, in said one of the casings there can be mounted a valve block comprising a valve for the brake unit to be built in said one of the casings and a valve for the hydraulic jack. A fluid pressure supplied through a common piping material to be connected to the valve block can thus be inputted to the brake unit and the hydraulic jack via each of the valves. However, since the other of the casings moves relative to said one of the casings, if a valve for the second brake unit to be built in the other of the casings is contained in the valve block mounted on said one of the casings, the valve block and the second brake unit have to be connected via a flexible hose. This brings about a poor control response of the second brake unit and a poor durability of the flexible hose due to a fatigue by extension and contraction as a result of the relative movement of the other of the casings.
In this case, it may be considered to mount the valve for the second brake unit on the other of the casings. In this arrangement, however, the piping material to be connected to a pressure source must be disposed separately on the side of one of the casings and on the side of the other of the casings, resulting in a troublesome laying out of the piping.
Further, in the above-described conventional actuator, since the inching is made by a pitch corresponding to the stroke of the hydraulic jack in either of the forward and backward movements, the inching pitch for the forward movement and the inching pitch for the backward movement become equal to each other. It follows that, when the actuator is used as a driving apparatus for positioning the workpiece receiver on a jig, the workpiece receiver can be changed in position only by an integer multiple of the inching pitch. Therefore, the freedom in the change in position is lost.
In addition, in the above-described conventional example, the actuator is constituted by two casings which move relative to each other. It is therefore only part (one of the two casings) of the actuator that can be utilized as the member for fixing thereto a driven object such as the workpiece receiver or the like. As a result, the rigidity and the accuracy of positioning of the driven object is impaired.
In view of the above-described disadvantages, the present invention has an object of providing an inchworm type of actuator in which, by enabling the inching movement with the use of a single casing, the layout of the piping material is simplified and the rigidity and accuracy of positioning of the driven object is secured, and the pitch in the forward movement and the pitch in the backward movement can be made different from each other.